Yuanhe Xingzuan "The register of the great families from the Yuanhe reign (806-820)" vol. 6, the Yuwen part 2 records:

(宇文)本遼東南單于之後, 有普回因獵得玉璽, 以為天授. 鮮卑俗呼天子為宇文, 因號宇文氏.

(The Yuwen) originally were the descendants of the southern Shanyu (of the Xiongnu). Someone within them called Puhui got a jade seal when he was hunting. This was regarded as a sign of imperial enthronement from heaven. According to Xianbei tradition, the son of the heaven was called the Yuwen. Thus (Puhui) called himself the Yuwen.

Yuwen were descendants of the nomadic Xiongnu who became assimilated with the Xianbei after 89 CE and ruled the Kumo Xi and Khitan (both Mongolic peoples) before being defeated by Murong Huang in 344 CE, upon which Yuwen separated from the Kumo Xi and Khitan. The language of the Yuwen is thought to be Turkic or a very distant branch of Mongolic.

Yuwen Mohuai of the Xiongnu was from Liaodong, the region beyond the northern border. His ancestor was a remote relative of the southern Shanyu. (The Yuwen) had been the chief of the eastern section (of the Xianbei) for many generations. The (Yuwen)'s language differed widely from the Xianbei's. (The Yuwen) people all had shaved hair, but the hair on top of the head was left as a decoration. When the hair was over some cun long, it would be cut short. The women had long robes, which draped from their shoulders to their feet, but (they) did not wear skirts. When autumn came, they collected Wutou as poison, and used it to shoot birds and beasts.